Human activities contribute to climate change by causing changes in Earth’s atmosphere in the amounts of greenhouse gas­es, aerosols (small particles), and cloudiness.

Human activities like burning of fossil fuels (coal, oil and natural gas), deforestation and various agricultural and industrial practices, are altering the composition of the atmosphere and contributing to climate change.

These human activities have led to increased atmospheric concentrations of a number of greenhouse gases (GHGs), including carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and ozone in the lower part of the atmosphere.

The largest known contribution comes from the burning of fossil fuels, which releases carbon dioxide gas to the atmosphere.

GHGs affect climate by altering incoming solar radiation and out-going infrared (thermal) radiation that are part of Earth’s energy balance. Changing the atmospheric abundance or properties of these gases and particles can lead to a warming or cooling of the climate system.

Since the start of the industrial era (about 1750), the overall effect of human activities on climate has been a warm­ing influence. The human impact on climate during this era greatly exceeds that due to known changes in natural processes, such as solar changes and volcanic eruptions.

Human activities result in emissions of four principal green­house gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and the halocarbons (a group of gases containing fluorine, chlorine and bromine).

These gases accumulate in the atmosphere, causing concentrations to increase with time. Significant increases in all of these gases have occurred in the industrial era (IPCC, 2007).

Carbondioxide is produced when coal, oil and natural gas (fossil fuels) are burned to produce energy used for transportation, manufacturing, heating, cooling, electricity generation, and other applications.

The use of fossil fuel currently accounts for 80 to 85% of the carbon dioxide being added to the atmosphere. Land use changes, e.g., clearing land for logging, ranching, and agriculture, also lead to carbon dioxide emissions.

Deforestation also releases CO2 and re­duces its uptake by plants. CO2 is also released in natural processes such as the decay of plant matter. Vegetation contains carbon that is released as CO2 when the vegetation decays or burns.

Over the past several hundred years, deforestation and other land use changes in many countries have contributed substantially to atmospheric CO2 increase.

Most of the net CO2 emissions from deforestation are currently occurring in tropical regions. Land use changes are responsible for 15 to 20% of current carbon dioxide emissions.

Methane (natural gas) is the second most important of the greenhouse gases resulting from human activities.

It is produced by rice cultivation, cattle and sheep ranching, and by decaying material in landfills. Methane is also emitted during coal mining and oil drilling, and by leaky gas pipelines.

Human activities have increased the concentration of methane in the atmosphere by about 145% above what would be present naturally.

Nitrous oxide is produced by various agricultural and industrial practices. Nitrous oxide is also emitted by human activities such as fertil­izer use and fossil fuel burning.

Human activities have increased the concentration of nitrous oxide in the atmosphere by about 15% above what would be present naturally.

Halocarbon gas concentrations have increased primarily due to human activities. Natural processes are also a small source. Principal halocarbons include the chlorofluorocarbons (e.g., CFC-11 and CFC-12), which were used extensively as refrig­eration agents and in other industrial processesण्.

Their presence in the atmosphere was found to cause stratospheric ozone depletion. The abundance of chlorofluorocarbon gases is decreasing as a result of international regulations designed to protect the ozone layer.

Ozone in the troposphere, i.e. in the lower part of the atmosphere, is another important greenhouse gas resulting from industrial activities. Ozone is continually produced and destroyed in the atmosphere by chemical reactions.

In the tro­posphere, human activities have increased ozone through the release of gases such as carbon monoxide, hydrocarbons and nitrogen oxide from motor vehicles and power plants, which chemically react to produce ozone.

The halocarbons released by human activities destroy ozone in the stratosphere and have caused the ozone hole over Antarctica.

Water vapour is the most abundant and important greenhouse gas in the atmosphere. However, human activities have only a small direct influence on the amount of atmospheric water vapour.

Human activities influence water vapour through CH4 emissions, because CH4 undergoes chemical destruction in the strato­sphere, producing a small amount of water vapour.

Sources of Agricultural greenhouse gas emissions

Agriculture contributes to about 20 % of the GHG emissions globally, today. Agricultural GHG emissions come from several sources.

1. Agricultural soil management

These are nitrous oxide emissions and account for about 60 percent of the total emissions from the agricultural sector. Nitrous oxide is produced naturally in soils through the microbial processes of nitrification and de-nitrification.

During nitrification, ammonium (NH4) produces nitrates (NO3,). During de-nitrification, nitrates (NO3) are reduced to nitrogen gas (N2). An intermediate step in both of these processes is the creation of nitrous oxide (N2O).

The large increase in the use of nitrogen fertilizer for the production of high nitrogen consuming crops like corn has increased the emissions of nitrous oxide.

Although nitrogen fertilizer is essential for profitable crop production, the development of practices for more efficiently using nitrogen fertilizer has the potential to significantly reduce nitrous oxide emissions while also reducing production costs and mitigating the nitrogen contamination of surface and ground waters.

2. Enteric fermentation

Methane is produced as part of the normal digestive processes in animals. During digestion, microbes in the animal’s digestive system ferment feed.

This process, called enteric fermentation, produces methane as a by-product which can be emitted by the exhaling and belching of the animal. Because of their unique digestive system, ruminant animals (e.g. cattle) are the major emitters of methane.

Beef cattle account for about 70 percent and dairy cattle for about 25 percent of these methane emissions. If beef and dairy cattle numbers increase, methane emissions will also increase. Feed quality and feed intake influence the level of methane emissions. In general, lower feed quality and higher feed intake lead to higher methane emissions.

3. Manure management

Methane is produced by the anaerobic (without oxygen) decomposition of manure. When manure is handled as a solid or deposited naturally on grassland, it decomposes aerobically (with oxygen) and creates little methane emissions. Manure stored as a liquid or slurry in lagoons, ponds, tanks or pits, decomposes anaerobically and creates methane emissions.

Dairy cattle and swine produce about 85 percent of the methane emissions. Methane emissions will increase as the number of large scale livestock confinement systems increases. Methane emissions can be reduced through the application of technologies designed to capture the methane and use it as an energy source.

4. Carbon dioxide from fossil fuel consumption

The use of fossil fuels in agricultural production accounts for eight percent of the emissions from agriculture. These emissions are primarily from combustion of gasoline and diesel fuel. Using renewable fuels can reduce the CO2 emissions from agriculture production.

5. Others sources

A variety of other sources produce greenhouse gas emissions. For example, most of the world’s rice is grown on flooded fields, which prevents atmospheric oxygen from entering soil. When rice is grown with no oxygen, the soil organic matter decomposes under anaerobic conditions and produces methane that escapes into the atmosphere.

Opportunities to reduce GHG emission in agriculture

Sequestering carbon in agricultural soils by reducing tillage

Reducing nitrous oxide emissions through more efficient use of nitrogen fertilizer